The present invention relates to a waveguide connecting method and structure for connecting waveguides to each other and, more particularly, to a waveguide connecting method and structure for connecting waveguides to each other by using a shim which closes a gap formed between the end faces of the waveguides.
When connecting waveguides to each other, if a gap is formed between the end faces of the waveguides or the flange surfaces of flanges formed on the ends of the waveguides, a reflection wave is generated at the connecting portion, and a loss (reflection loss) due to the reflection wave increases. In order to improve the reflection characteristics by decreasing the reflection loss caused at the waveguide connecting portion, a choke flange is generally used.
If the flange surfaces outside a choke groove cannot be brought into tight contact with each other, a sufficient effect cannot be obtained. A waveguide connecting structure for obtaining better reflection characteristics is proposed in Japanese Patent Laid-Open No. 9-312501 (reference 1).
FIG. 8 shows the sectional structure of the connecting portion of two waveguides 110 and 120 disclosed in reference 1. Referring to FIG. 8, a ring-like groove 116 is formed in the surface of a flange 112 of the waveguide 110 to surround an opening 111a. A thin metal plate 117 with spring properties and a radio wave absorber 118 are disposed in the groove 116. The metal plate 117 is bent to have an uneven section, and some bent portions 117a and 117b project from its flange surface 112a. 
When the waveguide 120 is to be connected to the waveguide 110, the bent portions 117a and 117b are forced backward as they are pushed by a flange surface 122a of the waveguide 120. Hence, when connecting the waveguides 110 and 120, the bent portions 117a and 117b of the metal plate 117 come into tight contact with the flange surface 122a of the waveguide 120. At this time, during connection, even if a gap is formed due to damage and unevenness of the flange surfaces 112a and 122a of the waveguides 110 and 120, it is closed midway by the bent portions 117a and 117b. 
In the conventional waveguide connecting structure described above, the position where the gap is closed by the metal plate 117 of the waveguide 110 is away from the opening 111a of the waveguide 110 and an opening 121a of the waveguide 120, that is, from the interiors of the waveguides. Since the discontinuity of the connecting portion itself of the waveguides 110 and 120 is not solved, a sufficient effect cannot be obtained in improving the reflection characteristics.
It is an object of the present invention to provide a waveguide connecting method and structure that can provide good reflection characteristics when waveguides are connected to each other.
In order to achieve the above object, according to the present invention, there is provided a waveguide connecting method comprising the steps of fabricating a shim with a cylindrical portion and a flange which projects from one end of the cylindrical portion outwardly, the cylindrical portion having an outer diameter substantially equal to an inner diameter of a first waveguide which is to be connected to a second waveguide, inserting the other end of the cylindrical portion of the shim into the first waveguide, and urging the second waveguide against the first waveguide, with an end face of the second waveguide being in contact with the flange of the shim, until the end face of the second waveguide abuts against an end face of the first waveguide.